Carrier suppression modulation



Jan. 22, 1929.

R. K. POTTER CARRIER SUPPRESSION MODULATION Filed Sept. 25, 1927 GridPotential INVENTOR. I RKPoLer BY L ATTORNEY Patented Jan. 22, 1929.

UETED STATES PATENT oFFics.

RALPH K. POTTER, OF NEW YORK, N. Y., 'ASSIGNOR T AMERICAN TELEPHONE ANDTELEGRAPH COMPANY, A CORPORATION OF NEW YORK.; I

CARRIER surrnnssron MODULATION.

Application filed September 23, 1927. Serial No. 221,554.

This invention relates to translating systems, and more particularly tomodulator arrangements, and is applicable, for instance, to signaltransmitting systemsmore particularly, radio transmitting systems-inwhich it is often desirable to modulate between carrier currents and thesignaling currents to produce in the output of the translating deviceside-bands, which side-bands (or one of them) may be selected and transmitted to the distant station, the carrier frequency component beingsuppressed.

The principal object of the applicants invention is to modulate, and tosuppress the carrier, by a method which is simpler than any heretoforeemployed.

In general, the applicant uses a single translating device having'certain characteristics and so adjusts the device that it will functionas a carrier suppression modulator, without the requirement of outsidebalancing means, modulated harmonic products of the impressed radiofrequency wave being selected in the output of the device instead of theside-band products of the fundamental radio frequency wave.

The principal advantage of the applicants method is that it permits theaccomplishment of carrier suppression modulation in a single translatingdevice. A further advantage is that with the use of the novel method,the modulator may functionalso as a frequency multiplier, this frequencymultiplication being desirable in certain types of radio transmissionsystems, particularly in connection with the use of the shorterJwavelengths.

These and other advantages of the invention will appear more clearly inthe follow-- ing detailed description.

The description of the invention is t0 beread with reference to theaccompanying drawing. Figure 1 of the drawing shows diagrammatically thecharacteristic of the translating device employed, and indicates thewaves in the output circuit resulting the input energy applied to thedevice and from variation of the input energy. Fig. 2 showsdiagrammatically the form of the voice wave in the input and theresultant output of a translating system used for the praC- tree of theapplicants method and the interrelation of the voice input and theoutput. Fig. 3 shows diagrammatically the singlevacuum tube system whichmay be used for the practice of the invention.

ing the characteristic described above. The 'characterlstic of such atube'is shown 111 Fig.

1 as a b c, the grid potential being plotted against plate current inthe diagram. If the grid biasing voltage is brought to the point N whichcorresponds with the symmetrical mean of the tube characteristic at b,and if the radio frequency represented by the wave W is impressed'on theinput of the tube, there will be no even'harmonics of the radiofrequency-or practically nonein the tube output. This may be shownmathematically as follows:

A curve which is symmetrical about a point (as is the vacuum tubecharacteristic shown in Fig. 1) may be represented mathematical- .ly bya series of odd powers as,

f(m)=am+bw +cw5+dw (1) Assuming the wave W to be impressed upon thistube to vary according to the re lation,

W =A sin pt, (2) and substituting W for 0'0 in equation (1) 9 we have, I

f(a a(A sin pt) b (A sin pt) c (A'sin' pt) (3) The expansion of thisexpression gives,

f(x) =aA sin pt+ 3 sin pt-%- sin 3 pt Equation (4:) contains only theodd multiples of the fundamental frequency of the wave Vi so that thesecond harmonic-in fact any of the even harmonics is not'present in thetube output.

With such adjustment of the grid biasing voltage, the output of the tubemay be represented by Wave B which, it is assumed, contains none of theselected harmonic to which the output circuit is to be tuned.

If, now, a low frequency voice wave W is applied to the input, the meangrid potential will be changed from the normal at N to the more positiveposition 0 and the less positive position A, alternately. Vilith thisvariation of the mean grid voltage, distortion occurs and theunsymmetrically distorted waves H and H will appear in the output. Ifthese waves H and H, are analyzed, it will be found that the componentsare h and (Z and it, and (Z respectively. In the case of wave H alrepresents the fundamental component and ]b1 the second harmoniccomponent. Likewise, in the case of wave H (Z and k represent thefundamental and second harmonic components, respectively. It will beunderstood, of course, that the wave form in the output varies from B toH and from B to H, as the mean grid voltage increases or decreases fromthe normal. From the above analysis ofthe waves H and H it appears thatalthough the fundamental frequency content of these waves suffers noreversal of phase as the grid bias is changed from A. to C and from C toA, the even harmonics of the fundamental carrier frequency undergo areversal in phase of 180 degrees. It will be noted further that theamplitudes of these waves pass through zero at-the sym metrical mean ofthe'characterlstic a b 0. From the above d1scuss1on, 1t Wlll be understood that if a single three-electrode vacuum tube is employed and ifthe grid voltage is adjusted as described above, the second harmonic ofthe improved radio frequency being selected, the resultant output of thetranslating device will be represented by a wave in some such form asthe lower Wave of Fig. 2, the upper wave of this figure representing themodulating voice wave. Here, it will be noted that the amplitude of themodulated wave passes through zero when the amplitude of the modulatingwave is zero. Also, as will be noted, at the point at which themodulated wave passes through zero, it undergoes a phase reversal. Theseconditions will be recognized as conditions characteristic of a carriersuppression modulator output.

In Fig. 3 of the drawing there is shown diagrammatically the apparatuswhich will function in. accordance with the applicants invention as asingle tube carrier suppres sion harmonic modulator. The arrangementdiffers from that of a simple grid modulating system only in that thegrid of the tub-e V is biased so that its mean potential corresponds tothe symmetrical mean of the tube characteristic for the particularharmonic selected, this being accomplished by adjustment of the batteryS, and in that a filter is placed in the output circuit which passesonly the sidebands of the selected harmonic.

Let it be assumed that the harmonic selected is the second. From theoscillator a carrier frequency f is impressed on the inputv of thetranslating circuit. Also the voice fre: quency f is impressed thereon.The output of the filter in the plate circuit of the tube V will then,in accordance with the method described above, be 27 plus or minus f Thecarrier f is modulated with the voice f and what is passed is theside-bands of the second harmonic of the carrier frequency f... If, forexample, in equation (1) we substitute for m,

a} (A sin pt-tB cos wt) (5) where A sin pt represents the high frequencyWave W and B cos wt represents the audio frequency wave, there results aseries of terms which may be tabulated as follows:

, (A sin pt) (6) I (A sin pt) (B cos wt). (6) (A. sin pt) (B cos wt) (6)(A sin pt) (B cos wt) (6) etc.

ponent is, however, small compared to the amplitude of the undistortedside-band as in any of the well known modulation circuits. In themathematical discussion of ordinary modulation circuits these terms areusually treated as negligible and disregarded.

The distinction between the applicants single tube carrier suppressionmodulator and the duplexcarrier suppression modulator, now well known inthe art, is to be understood. In the duplex device, the carrier is thefundamental high frequency impressed on the input circuits, and thiscarrier is actually present in both sides of the duplex circuit output.The output circuit, however, isso aranged that the carrier component inone side is neutralized by the carrier component in the other side, inthe associated circuitw iich carries the resultant output of the system.What is selected i the side-bands of the fundamental high frequencyimpressed on the input.

In the applicants arrangement, on the other hand, it is not themodulation between the low frequency and the fundamental high frequencyimpressed on the input which is used, but the modulation between the lowfrequency and one even harmonic of the fundamental high frequency. Thecarrier is not the fundamental but an even harmonic; and, with the tubeadjusted as described above, this harmonic carrier does not-at least inpractical amountappear at all in the system. It is suppressed by theneutralization due to the action of the tube; since it does not appearin the output, there is no need to balance it out. That is selected inthe output is the side-bands of an even harmonic of the fundamentalimpressed high frequencywhich harmonic component is suppressed ornon-existent, so far as appearance in the circuits is considered).

It is to be understood that the above detailed description serves thepurpose of clear illustration, whereas the scope of the invention isdefined in the appended claims.

What is claimed is 1. The modulation method which consists in impressinga high frequency wave and a low frequency wave on a translating device,suppressing a harmonic of the high frequency, and selecting theside-band products of the harmonic frequency and the impressed lowfrequency.

2. The method of wave transmission which consists in generatingoscillations of a radio.

frequency and oscillations of an audio frequency, impressing both.- theradio frequency and the audio frequency oscillations on a modulatingdevice, adjusting the modulating device so that no even harmonics of thefundamental radio frequency appear in the output of the device, andselecting the side bands of one even harmonic of said frequency.

3; The method of modulation which conslsts 1n lmpresslng on atranslating device oscillations of a radio frequnency and oscillationsof another frequency, adjusting the translating device so that no evenha monics of the fundamental radio frequency appear in the output of thedevice, and passing only the side-bands of one even harmonic of saidfrequency.

4. The method of producing an electric current wave of high frequencywith groups of amplitude pulsations corresponding to the pulsations of amodulating wave, which consists in generating oscillations of a highfrequency, impressing the high frequency oscillations and the modulatingwave on a translating device, adjusting the translating device so thatno even harmonics of the funda mental high frequency appear in theoutput of the device, and selecting the side-bands of one even harmonicof said frequency.

5. The method of carrier suppression modulation which consists inimpressing oscillations of a radio frequency and the modulatingoscillations on a translating device, ad]ust1ng the translating deviceso that no even harmonics of the fundamental radiov requency appear inthe output ofthe device, and selecting the sidebands of one even.-armonic of said frequency.

6. In a translating circuit, a translating device having a non-linearbut symmetrical characteristic, means for impressing oscillations of ahigh frequency on said translating device, means for impressingoscillations of another frequency thereon, means for impressing on saidtranslating device an auxiliary voltage corresponding to the point ofsymmetry of the characteristic, whereby no even harmonics of the highfrequency appear in the output of the device, and means for selectingthe side-bands of one even hareven harmonic of the high frequency.

8. In a translating system comprising a translating device'having anonlinear but symmetrical characteristic, an input and an outputcircuit, and sources of radio frequency and audio frequency energyassociated with the input circuit, the method of producing a modulatedoutput with the carrier suppressed, which consists in impressing theradio frequency and the audio frequency energy on the input circuit,impressing on said circuit an auxiliary voltage of such value that asthe mean input voltage varies with the Variation of the audio frequencyenergy a certain harmonic of the radio frequency in the output circuitWill undergo a reversal of 180 and the amplitude of the harmonic WaveWill be zero when the amplitude of the audio frequency Wave is Zero, andselecting in the output circuit the modulated certain harmonic of theradio frequency.

9. In a translating system comprising a three-electrode vacuum tubehaving a nonlinear but substantially symmetrical characteristic, aninput and an output circuit therefor, and sources of radio frequency andsignal. energy associated with the input circuit, the method ofproducing a modulated outputith the carrier suppressed, Which consistsin impressing the radio frequency and the signal energy on the inputcircuit, biasing the grid of the tube so that its normal mean potentialcorresponds to the symmetrical mean of the characteristic of the tubefor a given harmonic of the fundamental radio frequency, and selectingin the output circuit the modulated given harmonic.

10. A translating system consisting of a single three-electrode vacuumtube having a non-linear but substantially symmetrical Characteristic,an inputcircuit and an output circuit therefor, means for impressing ahigh frequency alternating voltage on said input circuit, means forimpressing on the grid of said tube a biasing voltage such that thenormal mean grid potential corresponds to the symmetrical mean of thecharacteristic for a given harmonic of the carrier frequency, and meansin said output circuit for selecting the modulated iven harmonic.

In testimony WhereoI, I have signed my name to this specification this10th day of September, 1927.

RALPH K. POTTER.

